Add note regarding the update status of special Docker tags

[yubiuser]

What does this PR aim to accomplish?:

Add note regarding the update status of special Docker tags.

This is how it looks like when it fails

pi@s740:~$ echo "nightly" | tr -d '[:alpha:]' | awk -F. '{ printf("%d%03d%03d%03d\n", $1,$2,$3,$4); }'
0000000000
pi@s740:~$ echo "v6-pre-alpha" | tr -d '[:alpha:]' | awk -F. '{ printf("%d%03d%03d%03d\n", $1,$2,$3,$4); }'
6000000000

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